Let $\displaystyle F$ be the collection of closed intervals $\displaystyle A_n=[1/n, 1-1/n]$ for $\displaystyle n=3,4,5,...$. What do you notice about $\displaystyle \bigcup F$? Is it closed, open, both, or neither?

I'm afraid the wording and available material has got me stumped on this, and being sick at the moment doesn't help.